The silicon controlled rectifier, or thyristor, has been the major device used in power applications for almost three decades. Its internal current latching characteristic that allows for the handling of heavy overloads without destruction has resulted the use of thyristors at energy ratings far higher than any competitive semiconductor device.
Thyristors need to be turned off by an external means such as a device that reverses load current or device terminal voltage. For inverter applications, an impulse signal controlled by an auxiliary device is the most common means to turn off a thyristor. In spite of the complication requiring auxiliary commutation circuits, thyristor inverters have dominated the high power applications until the last five years.
The Gate Turn-Off Thyristor or GTO is the preferred device for inverters and converters for power rating above 500 KW. Since it can be controlled with a low energy gate pulse signal, the commutation means used with conventional thyristors, a GTO inverter pole circuit in its simplest form contains only two GTOs and two anti-parallel diodes and snubber circuits (snubbers) to control voltage and current. The GTO is now being applied almost universally as it does not need the high energy pulse turnoff circuit. Instead, the turnoff pulse signal is applied to gate off the GTO. GTO's are available in power ratings comparable to thyristors and, manufacturers are working on even higher power GTO's.
As with any device, there are limitations imposed by current and voltage changes on GTO's that require the use of snubber circuits. These circuits are one of the principle sources of parasitic losses which limit inverter pole circuit efficiency. In applying GTO's to applications previously handled by thristors, little has usually been done except remove the commutating circuits and increase the size of the snubber circuit elements. The losses of the commutation circuits has been replaced by those in the snubber and gate circuits. There have been some circuits published for reducing snubber losses, but they use added GTO's and require high voltage elements.
For example, a high powered GTO pole circuit in the multi-kilovolt range has snubber circuit elements that dissipate 1% to 2% of the full power output. Thus, a pole circuit using a typical GTO snubber circuit would dissipate 10 to 20 KW for a one MW output. Losses of 1% to 2% are for GTO pole circuits running at most at 180 Hz. Pulse width modulation cannot be used as a means to reduce harmonics with a simple GTO pole circuit design without very high losses.
In high power circuits, the problem of removing the heat due to the snubber circuit losses dictates the cooling system and construction specifications. Since the size of the capacitors and inductors associated with the snubber circuitry depend on the GTO parameter limitations, only energy recovery is available as a means for significant loss reduction. The energy must either be transferred to the load or back to the power supply on the DC bus.